Photo by Hayarina
Many animes are based one Manga. This type of anime includes DRAGON BALL, Card Caprtor SAKURA, DEATHNOTE, Doraemon... Nowadays, I think more than half of animes on air are originally started as Manga.
Other source of amines are game and light novel, which are particularly increasing. I'd like to talk about "light novel" today.
Light Novel(juvenile novel) is basically novels for teens. Though the definition is not clear, it can be said they usually have many illustrations in the novel, in some cases by famous dojin illustrators. This may be because the readers are tend to be otaku...But the influence of light novel isn't limited to otaku; Quite frequently some light novel are listed as #1 sales among all novels.
This category is not so new, there have been since 1990 or more past. The early examples of anime introduced from light novel would be Slayers(1995). However, the number of animes seems to be exploded in this three years. Namely, Suzumiya Haruhi, Shakugan no Shana, Full Metal Panic... it would be impossible to show them all.
I'm frequent reader of light novel from my younger times, I grew up with Slayers and Records of Rodoss War. Usually these novels are written by plain text, however some have inpressive expression. My definite recommend is "Iriya no Sora, UFO no Natsu" or "Iriya's Sky, Summer of the UFOs" by Akiyama Mizuhito. Though his novel also transfered to anime, it seemed to me that the sense of the text was not fully transported. Full translation might be impossible by today's level of anime.
The distinction between light novel and common novel is becoming more vague. Some writers started their career in light novel have got most respected prize in literature (Naoki prize). In the othre case, Otsu-ichi(he's one of my favorite, also light novel writer at first) is now considered as one of the most prominent young mystery writer.
We can't take our eyes from light novel!